1. Field of Invention
The invention relates generally to a method and system for controlling amplifier power in an optical communications network.
2. Description of Related Art
Wavelength division multiplexing (WDM) has been used to increase the capacity of existing fiber optic networks. In a WDM system, plural optical signal channels are carried over a single optical fiber with each channel being assigned a particular wavelength. Such systems typically include a plurality of receivers, each detecting a respective channel by effectively filtering out the remaining channels.
Optical channels in a WDM system are frequently transmitted over silica based optical fibers, which typically have relatively low loss at wavelengths within a range of 1525 nm to 1580 nm. WDM optical signal channels at wavelengths within this low loss “window” can be transmitted over distances of approximately 50 km without significant attenuation. For distances beyond 50 km, however, optical amplifiers are used to compensate for optical fiber loss.
Optical amplifiers have been developed which include an optical fiber doped with erbium known as erbium-doped fiber amplifiers or EDFAs. The erbium-doped fiber is “pumped” with light at a selected wavelength, e.g., 980 nm, to provide amplification or gain at wavelengths within the low loss window of the optical fiber. Other types of optical amplifiers include erbium-doped waveguide amplifiers (EDWA), semiconductor optical amplifiers (SOA).
When optical amplifiers are cascaded in series along a transmission span, noise generated at each amplifier degrades the signal to noise ratio. FIG. 1 is a block diagram of a conventional optical communications network having a plurality of amplifiers 101, 102 and 103 positioned along transmission fiber 121, 122 and 123. The output of each amplifier includes noise in the form of amplified spontaneous emissions (ASE) and at least one signal as shown in FIG. 1. It is understood that a WDM system may carry multiple signals on separate channels and a single signal is shown for ease of illustration.
As is known in the art, at each amplifier stage the ASE increases due to the amplification of ASE input to the amplifier and ASE added at the amplifier. If, however, the output power of the amplifiers 101, 102 and 103 are equal, then the signal component is decreased to accommodate for the increase in ASE. For example, if the total output power of each amplifier is 8 mw, the power available for the signal is reduced as the ASE power increases from one amplifier to the next. As shown in FIG. 1, setting the output power of each amplifier equal results in a decreased signal-to-noise ratio (SNR) as the signal passes through multiple amplifiers.